Lucius Lilac vs Pilgrim Grey

How close are they?

ΔE 27.1, loose: different enough that you should pick on purpose. Pilgrim Grey is noticeably darker, much more muted than Lucius Lilac.

Can I use Pilgrim Grey instead of Lucius Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Pilgrim Grey is noticeably darker, much more muted than Lucius Lilac.

Can I use Lucius Lilac instead of Pilgrim Grey?

The same difference applies the other way round: Lucius Lilac is noticeably lighter, much more saturated.

ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.